Add multiple UrlGenerator classes for forum/api/admin

Spent quite a while looking into the best solution here and ended up going with three separate classes. Thanks to @Luceos for the PR that got this rolling (#518). My reasoning is:

- The task of routing and URL generation is independent for each section of the app. Take Flarum\Api\Users\IndexAction for example. I don't want to generate a URL to a Flarum route... I specifically want to generate a URL to an API route. So there should be a class with that specific responsibility.
- In fact, each URL generator is slightly different, because we need to add a certain prefix to the start (e.g. /api)
- This also allows us to get rid of the "flarum.api" prefix on each route's name.
- It's still DRY, because they all extend a base class.

At the same time, I could see no reason this needed to be "interfaced", so all of the classes are concrete.

Goes a long way to fixing #123 - still just a few places left remaining with hardcoded URLs.

src/Http/RouteCollection.php

@@ -88,9 +88,13 @@ class RouteCollection
 
     public function getPath($name, array $parameters = [])
     {
-        $parts = $this->reverse[$name][0];
-        array_walk($parts, [$this, 'fixPathPart'], $parameters);
+        if (isset($this->reverse[$name])) {
+            $parts = $this->reverse[$name][0];
+            array_walk($parts, [$this, 'fixPathPart'], $parameters);
 
-        return '/' . ltrim(implode('', $parts), '/');
+            return '/' . ltrim(implode('', $parts), '/');
+        }
+
+        throw new \RuntimeException("Route $name not found");
     }
 }